This invention relates to a speed detector capable of detecting the speed of a linearly moving body without contacting the same, and more particularly to a speed detector constructed such that there is no limit of the range of movement of the body to be measured.
It has been almost impossible to directly detect the linear speed of a member actuated by an oil pressure driving device without contacting the member over a long stroke of several hundred milimeters. One example of a conventional contactless speed detector is illustrated in FIG. 1 of the accompanying drawing. The detector shown therein comprises a hollow cylindrical protective cover 1 of pure iron containing a detection coil 2 wound on a hollow bobbin, not shown, and held in a predetermined position with respect to the protective cover 1, and a rod shaped permanent magnet 3 having N and S poles and supported by a rod 3a at its one end. The magnetic flux produced by the magnet flows through the protected cover as shown by dotted lines 4.
As the magnet 3 is reciprocated by a moving body in the axial direction of the protective cover 1 into and out of the detection coil 2 a direct current voltage e will be induced across the terminals C.sub.1 and C.sub.2 of the detection coil having a magnitude proportional to the relative speed between the magnet and the detection coil and a polarity depending upon the direction of movement of the magnet. The voltage e is expressed by the following equation ##EQU1## WHERE L: the length of the protective cover 1
Lc: the length of the detection coil 2 PA1 .PHI.: the magnetic flux linking the detection coil PA1 U: the speed vector of a conductor (in this case the conductor of the coil 2) with respect to a fixed coordinate axes PA1 B: the density of the magnetic flux vector PA1 DL: the segment vector of the conductor along a path C EQU UBdL = U(BdL) = (U .times. B) .dL PA1 Bn: A component of vector B in a direction perpendicular to any closed curved surface S including the path C in the outer periphery thereof.
In the conventional speed detector shown in FIG. 1, as the magnet is supported in a cantilever fashion, the magnet tends to vibrate while it is reciprocated, thus causing the induced voltage e to fluctuate. Moreover as it is necessary to increase its effective stroke Lc (the length of the detection coil) it is necessary to make the length of the protective cover to be more than two times that of the effective stroke.